iOS: Discard the depth and stencil attachments of the framebuffer before...

iOS: Discard the depth and stencil attachments of the framebuffer before presenting the color renderbuffer. (#2737)

flow/instrumentation.cc

@@ -71,7 +71,7 @@ void Stopwatch::Visualize(SkCanvas& canvas, const SkRect& rect) const {
   const SkScalar bottom = y + height;
   const SkScalar right = x + width;
 
-  // Scale the graph to show frame times up to those that are 4 times the frame time.
+  // Scale the graph to show frame times up to those that are 3 times the frame time.
   const double max_interval = kOneFrameMS * 3.0;
   const double max_unit_interval = UnitFrameInterval(max_interval);
 

ui/gl/gl_surface_ios.mm

@@ -216,6 +216,14 @@ void GLSurfaceIOS::SetupFramebufferIfNecessary() {
 }
 
 bool GLSurfaceIOS::SwapBuffers() {
+  const GLenum discards[] = {
+    GL_DEPTH_ATTACHMENT,
+    GL_STENCIL_ATTACHMENT,
+  };
+
+  glDiscardFramebufferEXT(GL_FRAMEBUFFER, sizeof(discards) / sizeof(GLenum),
+                          discards);
+
   glBindRenderbuffer(GL_RENDERBUFFER, colorbuffer_);
   return [[EAGLContext currentContext] presentRenderbuffer:GL_RENDERBUFFER];
 }